1. Field of the Invention
The present invention relates to an air meter and a method of producing the same and more particularly but not exclusively relates to an air meter for an internal combustion engine of an automobile and a method
2. Prior Art
In the known air meter for an internal combustion engine, the air intake passage is, generally, constituted by two passages; a main passage and an auxiliary passage. A hot wire sensor is arranged in the auxiliary passage. An air meter which has an auxiliary passage arranged not to traverse the main passage is called a bypass type air meter. One typical example of the bypass type air meters is disclosed in U.S. Pat. No. 4,723,443, in which the auxiliary passage has first and second flow passages connected to each other in an L shape. The first flow passage is formed in parallel with the main passage and has a hot wire sensor disposed within it whereas the second flow passage is arranged perpendicularly to and arcuately around the main passage. In this air meter, the throttle body thereof is divided to form the second flow passage of the auxiliary passage, and an insulator is interposed between the divided parts. The second flow passage of the auxiliary passage has a shoulder portion at a location where the second flow passage communicates to the first flow passage. The shoulder portion has an enlarged width or diameter. Thus, a dead volume to the air flow in the auxiliary passage is formed at the shoulder portion.
U.S. Pat. No. 4,501,249 teaches a bypass-type air meter in which a second flow passage of an auxiliary passage is arranged radially and perpendicularly to a main passage.
Another earlier attempt is disclosed in Japanese Patent Application No. 62-148993 in which a second flow passage of an auxiliary passage is arranged to cross a main passage. This air meter is called an axial flow type air meter. In this axial flow type air meter, it is not necessary to divide the throttle body since the second passage traverses the main passage, and hence the air meter provides an advantage of the throttle body being made of one piece.
In the bypass-type air meter of U.S. Pat. No. 4,723,443, a dead volume, which a formed in the shoulder, causes a turbulence in the air flow and can involve a fluctuation of an output of the hot wire sensor and a considerable increase in S/N ratio.
U.S. Pat. No. 4,501,249 does not teach how to form the auxiliary passage of the bypass-type air meter. To form the L-shaped auxiliary passage in the throttle body, it is generally necessary to divide the throttle body and then to machine a predetermined portion thereof to form the auxiliary passage. In this case, a shoulder portion is liable to be formed at a connection portion between the first and second flow passages due to an error in machining. The shoulder portion produces a dead volume which involves the same problem as above stated.
In the axial flow type air meter of Japanese Patent Application No. 62-148993, the first and second flow passages of the auxiliary passage are formed by machining after the throttle body is formed by die casting. This machining operation is likely to produce misalignment at a corner portion or a connecting portion between the first and second flow passages. There is a considerably high possibility of a dead volume being produced, which incurs the same problem as in the other air meters of the prior art.